1. Field of the Invention
This invention relates to solid-state relays generally and, more particularly, to the drive circuits therefore.
2. Description of the Prior Art
Solid-state relays are widely used in a variety of applications formally the domain of electromagnetic relays. However, the speed at which the solid-state relay operates may be too slow for some uses. Usually the turn-on time of the relay is adequate (approximately 100 .mu.s or less) but the turn-off time can be an order of magnitude slower. An exemplary prior art form A (normally off) relay 1 is shown in FIG. 2.
The relay 1 uses series connected MOS transistors 2, 3 to selectively provide a conductive path between output terminals 4, 4'. Photodiode array (PDA) 6, when illuminated by LED array 7, provides the charge (voltage and current) necessary to drive transistors 2, 3 into conduction. Circuit arrangement 9 speeds discharge of the gates of transistors 2, 3 when the PDA 6 stops producing current. When the PDA 6 is producing current, the voltage drop across current sensing resistor 13 forces depletion mode JFET 14 to remain nearly non-conductive until the gates of transistors 2, 3 are fully charged. Then, in the steady-on state phase, the JFET 14 draws a small amount of current from the PDA 6. The resulting voltage drop across resistor 13 is approximately equal to the pinch-off voltage of the JFET 14, thereby leaving sufficient voltage on the gates of transistors 2, 3 to keep them on. When the PDA stops producing current, the voltage drop across resistor 13 decreases, allowing JFET 14 to conduct more heavily, thereby discharging the gates of transistors 2, 3.
While this arrangement allows for faster turn-off than just using a shunt resistor (not shown), it is not as fast as may be desired because as the JFET 14 conducts, charge remaining on the PDA 6 due to capacitance forces a voltage drop across the resistor 13, reducing the conductivity of the JFET 14.
Thus, it is desirable to provide a solid-state relay with faster turn-off time than previous relays.
It is a further desirable to provide a fast turn-off time relay without significantly impacting the turn-on time of the relay or external characteristics of the relay (e.g., LED 7 current) and without complex circuitry.